Examples of smoke detectors for the early detection of fires are ionization smoke detectors in which the change in conductivity of ionized air is utilized to indicate fire aerosols, and optical smoke detectors in which the absorption or scattering of light by smoke particles is used. Since in ionization smoke detectors radio activity - albeit low-level is present, optical smoke detectors are increasingly used, especially scattered-light detectors, since the latter can be of a construction having a small space requirement.
Optical smoke detectors operating on the scattered-light principle contain a radiation source and a radiation detector. The detector is arranged outside the direct radiation area of the radiation source, but, in the presence of smoke or fire aerosol in the radiation area (measuring chamber), the detector is exposed to scattered radiation and generates electrical output signals depending on the strength of the scattered radiation. These signals are evaluated in an electronic circuit present in the smoke detector for alarm generation, or they are passed to the central process unit. In order to avoid interference by outside light, the light sources frequently operate in a pulsed manner, for example see the smoke detector system described in EP-B1-0'079'010.
Fire alarm systems must be ready for operation over long time periods. The smoke detectors are exposed to the harmful effects of the surrounding atmosphere, e.g. dust or corrosive vapors. Furthermore the quality of the electronic components, especially the radiation source and the radiation detector, can be reduced by aging. It is therefore necessary to check the operational capability of the smoke detectors at regular intervals.
In practice, this checking is usually effected by igniting a small test fire underneath the smoke detector, to produce smoke which can enter the detector and make it respond. Testing also has been carried out by placing a burning wick, e.g. on a rod, directly under the detector (for example see U.S. Pat. No. 4,271,693). Apart from the fact that these methods are rather cumbersome, they frequently result in contamination of the detector, which could render it incapable of operation.
An attempt has therefore been made to replace the smoke by droplets of fluid, e.g. artificially-produced mist, since such aerosols affect the smoke detectors in the same way as smoke from fires. For example, a mist of water droplets has been produced and used for the test. The layer of water deposited on the inner surfaces makes the detector inoperative over long periods.
Test agents which have proved most successful are mixtures of halogenated hydrocarbons (propellants) which have a suitable boiling point and which are blown directly into the smoke detectors from suitable storage containers arranged in so-called detector testers (for example see DE-B2-20'54'027). Due to the pressure-loss during discharge a suitable quantity of aerosol is produced for testing the smoke detector. Due to the high vapor pressure of the halogenated hydrocarbons, the propellant evaporates within a short time and the operational capability of the detector is not impaired.
A suitable detector tester for the testing of smoke detectors with halogenated hydrocarbons consists of a housing open at one end, which can be placed over the smoke detector, whose volume is at least twice the volume of the smoke detector, and a container connected to the housing that contains the propellant liquified under pressure and which, with the housing in place, has a spray valve operated manually or automatically, whose nozzle leads into the inside of the housing.
Because of the environmentally-harmful properties of halogenated hydrocarbons, these can no longer be used. Materials considered as replacements are mostly inflammable, toxic, corrosive and/or expensive (see Nachr. Chem. Tech. Lab. 40 [1992], No. 12, page 1398).
Other known test methods for fire alarm systems with optical smoke detectors operate without the use of test gases. Generally speaking, in this case also procedures are used which simulate the ingress of smoke into the smoke detectors. Here for example, an additional light source which projects light directly onto the radiation detector can simulate the appearance of scattered light in the smoke detector (U.S. Pat. No. 2,627,064). A test device is described in U.S. Pat. No. 3,585,621, in which a calibration element which projects scattered light onto the light detector and, for example, simulates a smoke concentration of 4%, is used to check the light source. In GB-PS-1,079,929 testing of the optical smoke detector is implemented by simulating an alarm (scattered light) by introducing a vane into the radiation path.
The voltage at the input of the threshold detector can also be increased to a value just under the response voltage by means of a switch (JP-PA-46-12199); the peaks of the diffused stray light, which are normally well below the response threshold, are in this case increased until an alarm is generated during the test. Here it is possible to test the operation of flash lamp, photocell, amplifier and switching circuit simultaneously.
A photo-electric smoke detector for indicating both alarm and fault conditions is disclosed in U.S. Pat. No. 4,306,230, which has a detection device consisting of a light source and a light-sensitive element arranged outside the direct path of the light source, which generates an output signal in relation to an initial change caused by the presence of smoke. In the smoke detector a second detection device is provided, which makes it possible to detect a fault condition (contamination of the surfaces of the light source or light-sensitive element) by allowing a predetermined amount of light to fall through an opening in the housing of the detector onto the light-sensitive element. If the amount of light falling through the opening does not trigger a signal within a specific range, then a detector fault is indicated.
A method for testing photo-electric smoke detectors is described in EP-A1-0'122'5489, in which, in addition to the smoke-indicating light source and the smoke-indicating light detector, a test light detector which receives light directly from the light source, and a test light source which radiates light directly onto the smoke-indicating light detector in relation to the output signal of the test light detector, are provided in the measuring chamber of the scattered-light smoke detector. In this method the operation of the smoke detector is continuously monitored by a control center; the detector is tested to ascertain if it is operating correctly and if its sensitivity is within the normal range.
The disadvantage of all these test methods is that means for testing the detector have to be provided in each individual smoke detector, which makes the fire alarm system considerably more expensive.
A device for testing the operation of optical smoke detectors is described in JP-PA-53-99899, in which a part of the housing that shields the measuring chamber against the external atmosphere, is comprised of rubber or an elastic body, e.g. a sponge. The elastic body is covered by a flat plate which has an opening in the center. For testing, a device consisting of four arms that are placed over the detector, is used. In the center of the four arms is a needle that passes through the rubber into the measuring chamber of the detector and simulates the appearance of scattered light in the chamber. This means that additional structure must be provided on or in the detectors, to facilitate the function test.